SCALLOP. 193 



studded with glands which secrete a colouring matter exactly agreeing with 

 the tints on the exterior of the shell, which they assist in ornamenting. Be- 

 tween the leaves of the mantle are placed the branchiae or gills, always four 

 in number, formed of fringes pointing outwards, and free at their outer edges, 

 so as to float loosely in the surrounding water. The mouth is placed between 



FIG. 193. SCALLOP. 



the two innermost gills at the point where they unite ; it is a simple orifice, 

 guarded by four thin and sensitive lips. Between the layers of the mantle is 

 enclosed a fleshy organ, something resembling a tongue in its structure ; this, 

 though small in the scallop, becomes in many genera of large dimensions, and 

 assumes functions of great importance and interest. At the hinge or point 

 where the two shells are united, there is a very elastic substance, the resiliency 

 of which tends to force them apart. To counteract this, a stout, compact, and 

 very powerful muscle proceeds from near the .centre of one valve, to that of 

 the other, which, by its contraction, draws them together and keeps them 

 closed. When the animal wishes to open its shell, it relaxes this muscle, and 

 the elastic ligament, previously in a state of compression, forces them asunder. 

 It is the contractile power of this muscle which renders it so difficult to open 

 an oyster : the inserted knite cuts through the muscle, and it opens immedi- 

 ately. __ In a great majority of the Conchifera there are two of these muscles 

 placed far apart, as in the common mussel. The hinge also, in many, is 

 much more complicated, presenting a curious array of notches and teeth 

 depressions and elevations that lock into each other. In genera so constructed 

 the elastic ligament is placed upon the outside of the hinge, and opens the 

 shell by its contraction, not by its expansion. When approximated, the valves 

 of the shell generally fit accurately to each other, but in some cases a part of 

 the circumference is left permanently open. 



Whoever for a moment reflects upon the arrangement of the 

 branchial apparatus, and the position of the mouth, consisting 

 as it does of a simple aperture unprovided with any prehensile 

 organs, must perceive that there are two circumstances connected 

 \vith the economy of a Conchiferous Mollusk, and those not of 

 secondary importance, by no means easily accounted for. It is, 



13 



